OUTLINE: This is a multicenter study. Patients are stratified according to baseline polymerase chain reaction (PCR) level (< 3 logs below baseline vs ≥ 3 logs below baseline), time on imatinib mesylate (12 to < 24 months vs 24 to < 36 months), imatinib mesylate dose (< 400 mg vs 400 mg to < 600 mg vs 600 mg to 800 mg), and center. Patients are randomized to 1 of 2 treatment arms.